The overall goal of this research is to understand the auditory cortical mechanisms involved in spatial localization of complex sound. To achieve this goal, we have implemented an earphone delivery system that mimics, at the eardrum of a cat, sound coming from a particular direction in space. The stimulus set constitutes a virtual acoustic space. Using this approach to study directional sensitivity of primary auditory cortical neurons, we propose to pursue four related specific aims. 1) We plan to study the directional sensitivity of AI neurons over a range of signal intensity to test the hypothesis that directional selectivity, as expressed in the timing and discharge strength within a neuron's spatial receptive field, is intensity tolerant. We will do this under both monaural and binaural conditions. We will also study a neuron's directional sensitivity under reverberant conditions that are believed to be involved in determining sound distance. 2) We will study the cortical mechanisms that are involved in processing sound motion. In doing so we will separate the salient cues that give rise to the motion response of AI neurons. 3) We will test the hypothesis that directional sensitivity of AI neurons is relatively insensitive to competing background sounds, using a wide range of background sound condition. 4) Using a theoretical framework based on the theory of maximum likelihood estimation, we will test the upper limit of performance that can be expected from an ideal observer given the information contained in cortical spatial receptive fields. The results will be compared to human psychophysical performance.